1. Field of the Invention
The present invention relates to an image forming apparatus and a process cartridge.
2. Discussion of the Background
With the development and diffusion of home computers, the demand for improving the quality of color images produced by an electrophotographic image forming apparatus is extremely strong. There is nothing surprising for such a demand considering that materials containing color images taken by a digital camera or a scanner and half-tone colored plot areas for a graph are commonly used.
In addition, images taken by a digital camera can be also commonly developed not only in silver halide photography but also by a dye sublimation printer or an inkjet printer. However, these image formations take a long time and cost of paper and ink therefore is expensive. Therefore, to make a poster and a presentation material, the speed and cost of production cause problems.
The image formation using electrophotography is excellent in light of the production speed and cost but needs improvement on image quality. To improve the quality of images produced in the image formation using electrophotography, it is good to decrease the particle size of a toner. But, as the particle size of a toner decreases, cleaning performance by a cleaning blade for removing the toner remaining on an image bearing member becomes insufficient. As a result, the image quality significantly deteriorates. Since the amount of charge in a small toner particle per weight unit is large, the electrostatic force attracting the toner particle to an image bearing member increases. When a cleaning blade is used to scrape toner particles remaining on the image bearing member, the force of scraping small toner particles by the cleaning blade is relatively small in comparison with the case of scraping large toner particles since the contact portion between the small toner particles and the cleaning blade is correspondingly small. Further, small toner particles can slip through the cleaning blade since the friction between the image bearing member and the cleaning blade can cause the cleaning blade to vibrate. Consequently, the quality of images deteriorates. When a large amount of toner has slipped through the cleaning blade, the obtained resultant images may be abnormal images having streaks. In addition, such toner particles that slip past the leaning blade attach to a charging roller and make the resistance and voltage thereof uneven, which may result in the occurrence of white streaks in an obtained image.
To reduce the attraction force between toner particles and an image bearing member, it is effective to reduce the surface free energy of the image bearing member. For example, unexamined published Japanese patent application No. (hereinafter referred to as JOP) H10-69100 describes that an image bearing member having a surface free energy of not greater than 30 dyne/cm can be obtained by using a binder resin containing a fluorine resin. An image forming apparatus using this image bearing member can form quality images for a while but has a drawback that the surface free energy of the image bearing member increases over repetitive use, which leads to deterioration of the quality of obtained images.
JOP 2001-66812 describes an image forming apparatus using an image bearing member having a surface layer formed of amorphous silicon containing fluorine and a cleaning device which removes the material causing image flow which attaches to the surface of the image bearing member during repetitive image formation. The surface free energy of the image bearing member is restrained to be not greater than 40 mN/m. But since the surface layer of amorphous silicon containing fluorine is formed by a gas phase method, cost of the image bearing member is expensive. In addition, there is no specific description about the cleaning performance for removing the remaining toner on the image bearing member when small toner particles are used. In general, the cleaning performance deteriorates not in all the image formation area but in a limited part in a concentrated manner. This is ascribable to the form of a cleaning blade and variation of the surface free energy of an image bearing member. Therefore, it is not sufficient to measure the surface free energy of an image bearing member only at one point.
JOP 2001-272809 describes an image forming apparatus using an image bearing member having a siloxane based resin layer with a surface free energy of from 40 to 80 mN/m and a toner having an average particle diameter of from 4 to 12 μm and an average charging amount of from 10 to 30 μC/g. However, the toner for use in the image forming apparatus is set to have a low amount of charge on average to weaken the attraction force between the toner and the image bearing member. Thereby, the stability of the images obtained using this toner is relatively low in comparison with the case of when a typical toner is used. This is not preferred because the background fouling easily occurs depending on environment.
JOP H11-311875 describes an image forming apparatus using an image bearing member having a surface free energy of from 3 to 65 mN/m, in which the rise of the surface free energy is limited to 25 mN/m during the duration of the image bearing member. In the measurement of the surface free energy described in JOP H11-311875, three kinds of solvents, i.e., water, methylene iodide, and α-bromonaphthalene, are used. But it is not possible to evaluate the calculation error of the surface free energy when only three kinds of solvents are used. Water is especially vulnerable to measurement error and difficult to obtain the true surface free energy. In addition, as described above, it is not sufficient to measure the surface free energy at only one point on an image bearing member except when a user forms images totally at random. This is because the surface free energy tends to distribute when a user prints tables such as quotations and project protocols having definite forms in a large number. The distribution tends to occur not in the circumference direction but in the longitudinal direction. The distribution in the longitudinal direction is not preferred because the quality of images easily deteriorates due to the deterioration of the cleaning performance.
JOP H11-311875 also describes an image forming apparatus using an inexpensive organic image bearing member. However, in an image forming apparatus using such an organic image bearing member, the organic image bearing member is easily abraded by the friction between the organic image bearing member and a cleaning blade. Therefore, to obtain an organic image bearing member having a long life, it is desired to thicken the layer thickness of the organic image bearing member to allow for the decrease of the layer thickness due to the abrasion thereof. Naturally, the layer thickness significantly decreases as image formation is repetitively performed. Therefore, the electric capacitance of the image bearing member significantly changes after repetitive use thereof. It is thus difficult to make the image density constant. JOP H11-311875 further describes an organic image bearing member having a surface layer containing a fluorine compound. However, since the abrasion rate of the surface layer containing a fluorine compound is not significantly slow in comparison with the case in which a typical organic image bearing member is used, the surface layer still has a considerable thickness. But since too thick a layer hinders the transfer of positive holes, the voltage after irradiation and the remaining voltage tend to rise. Consequently, it is not suitable to use the organic image bearing member in an image forming apparatus for producing quality images.
To improve the anti-abrasion property of an inexpensive organic image bearing member, JOP H01-170951 describes an organic image bearing member containing a filler such as a metal oxide in the surface layer thereof. This image bearing member is preferred since the image bearing member has an extremely excellent anti-abrasion property. However, the surface free energy of this organic image bearing member rises during image formation, resulting in deterioration of transfer efficiency, which may lead to production of abnormal images having, for example, hollow defects. Further, there is another drawback that the cleaning blade is abraded over time so that the cleaning performance tends to deteriorate.
Japanese patent No. 2859646 and JOP 2002-229241 describe a technology in which lubricant materials externally added to toner particles are transferred (attached) to an image bearing member when an image is developed on the image bearing member with the toner during image formation. Thereby, the surface free energy of the image bearing member is reduced. This technology is extremely preferred because the friction between the image bearing member and a cleaning blade can be reduced and the cleaning performance for removing the remaining toner is secured. However, since the lubricant materials are supplied only to the developed portions on the image bearing member, the surface free energy of the non-developed portions is kept high. Therefore, when a user prints tables such as quotations and project protocols having definite forms in a large amount, the surface free energy of the image bearing member tends to significantly vary. The cleaning blade tends to vibrate at the border of an area having a high surface free energy and an area having a low surface free energy, which may lead to poor cleaning performance and squawky friction noise.